1. Field of the Invention
The present invention relates to a method of making a MESFET (Metal Semiconductor Field Effect Transistor) using a III-V compound semiconductor.
2. Description of the Related Art
Various types of field effect semiconductor devices using III-V compound semiconductors have been recently proposed. As one of the field effect semiconductor devices, for example, a MESFET with a Schottky gate electrode using a semi-insulating GaAs substrate is made by a process shown in FIGS. 1A to 1C. As shown in FIG. 1A, after a silicon oxide layer 2 having a thickness of 5000 .ANG. is formed on a surface of a semi-insulating GaAs substrate 1, openings 4 and 5 for source and drain regions are formed therein, using a resist layer 3 as a mask. Si ions are implanted into the exposed surface of the substrate 1 to provide source and drain regions 6 and 7 therein.
As illustrated in FIG. 1B, the silicon oxide layer 2 located between source and drain regions 6 and 7, which define a channel region, is removed, and the surface of substrate 1 is covered with a resist layer 8. The resist layer 8 is patterned to provide an opening 9 therein. Si ions are implanted again into the exposed surface of substrate 1 through the opening 9 to form a channel region 10 between source and drain regions 6 and 7. The substrate 1 is heat-treated to activate the implanted Si ions. As shown in FIG. 1C, ohmic electrodes 11 and 12 of an Au--Ge alloy are provided on source and drain regions 6 and 7, respectively, and Schottky gate electrode 13 of a Ti--Pt--Au layer is provided on the channel region 10, thereby obtaining a MESFET.
Since, however, the vapor pressure of As is high in III-V compound semiconductors, especially the GaAs substrate, careful consideration has been required for providing FETs such as MESFETs on the substrate. In such a MESFET, it is necessary to lower the impurity concentration of the channel region as compared with that of source and drain regions, and it is required to keep the depth of the channel region at approximately 0.1 .mu.m. A photolithography process is also required for forming these regions and electrodes. In the photolithography process, unwanted contamination due to resist residue is caused or the substrate surface is damaged by the heat-treatment. Accordingly, desired characteristics of the MESFET cannot be obtained. Furthermore, passivation must be taken into consideration to stabilize the characteristics of the device.